The overarching long-term goal of this research program is, and has been since its inception, a complete understanding of the mammalian magnocellular hypothalamo-neurohypophysial system (mHNS) as it senses and responds to the physiological demands placed on it. These demands arise from fluctuations in both the internal and external milieu. Since this oxytocin-(OT) and vasopressin- (VP) producing system exists in all mammalian brains, what we learn should have general applicability and importance in health and disease. One goal has been to identify the various cell types, neural and non-neural, involved in the homeostatic roles played by the mHNS, and to understand the processes and mechanisms by which these cell types interact. Consisting chiefly of the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei and their main axonal terminations in the pituitary neural lobe (NL), the mHNS has achieved model system status because it is well characterized physiologically (i.e., in blood pressure, temperature and water regulation, parturition and lactation/nursing) and has revealed much in investigations at many levels of analysis. So well known are the peripheral effects of mHNS outputs that even findings obtained in biophysical and molecular biological investigations using brain slices or cell cultures can most often be meaningfully related to functioning of the intact system. The work proposed here includes studies of the mHNS using microscopic, immunocytochemical, biochemical and physiological approaches as converging operations aimed at uncovering fundamental mechanisms of CNS function. Specific aims for the requested period of support are the following: Aim 1. Investigations of OT and VP neuronal coupling in the SON. Experiments will determine connexins involved and the effects on inter-neuronal coupling of activating receptors that are known to be important participants in the physiology of the mHNS. The potential influence on coupling of removing tonic inhibition in the SON will also be studied. Aim 2. Interactive roles of neurotransmitters and modulators in the control of neuronal excitability in the SON. Experiments will determine the influences of tonic receptor activation under basal vs. stimulated physiological conditions. Also to be examined are the effects on excitability of simultaneously activating two or more of the receptors that are known to be expressed in the SON.